I'm retired and have been so for many years, but for what it's worth, I feel that those who tell you they joined up on patriotic grounds, "to do their duty" etc., are romancing themselves, as of all my mates and acquaintances I don't think that was ever the reason they joined.*
In my own case, I was being a typical teenager and going though that rebellious stage where I wouldn't apply myself to my schoolwork (I was plenty capable enough), and I was giving my parents persistent low level grief, wagging school and otherwise making life miserable for all those in any sort of authority about me.
During a teacher/parent interview the headmaster of my school bought up the suggestion that what I really needed was a stint in the Military, however that was problematic as I was only 14 years old at the time. My parents agreed, although I'm sure that the idea disturbed them a bit,... so of course being the little b@rstard that I was, I immediately set my heart on it.
Now at that time the Royal Australian Navy had just introduced a new form of entry, the Junior Recruit, for boys aged 15 years. By the time I turned 15 I had probably driven my parents to distraction and they agreed to allow me to sit the entrance exam.
The rest is history,...
Other than that, I enjoyed my 12 years and had I not married would probably have stayed on until retirement.
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* Having said that, once I did join, I can honestly say that I willingly fulfilled my obligations and was very proud to serve my country, however I feel that "going on about it" is a sign of false pride. As far as I'm concerned, you sign on the dotted line, and then you do what is expected of you to the absolute best of your ability without complaint or the need or desire for recognition. If you can't do that, well,... you're prolly gunna be either, a bit disappointed, or a general pain in the @rse to all you come in contact with.